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How & Why Do Leaves Change Color?

Despite their astonishing record of losses when dealing with lumberjacks and beavers, trees are pretty tough customers. Their trunks, branches, roots and twigs are all more than capable of enduring a winter's worth of freezing temperatures, snow, sleet and hail. Their leaves, though? Eh, not so tough. The broad, thin leaves of a broadleaf tree (like a maple, an oak, a birch, or a poplar) are an Achilles' heel when winter comes, and are vulnerable to freezing and damage from the elements. In order to survive, the trees either have to somehow protect the delicate leaves or shed them.

Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season's vibrant colors.

Color Coding

Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis.

As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades

leaves-mapleYellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees' leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.

Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they're getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves' cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.

Brown: The humdrum color is the result of waste products trapped in the leaves.

That covers the basics of how each of the colors can be produced. But which color we ultimately see depends on several factors, such as"¦

Species: Certain colors are characteristic of particular tree species and can be used to help identify the type of tree you're looking at. Oak leaves turn red, brown, or russet, hickories turn golden bronze, poplars turn golden yellow, dogwoods turn a purplish red, beeches turn a light yellow/tan, birches turn bright yellow, sugar maples turn orange-red, black maples turn a glowing yellow, and red maples turn scarlet. Some trees, notably elms, don't go through much color change at all; there's just a dull brown and then the leaf is gone with the wind.

Weather: The temperature and moisture levels a tree is exposed to before and during the time its leaves' chlorophyll breaks down can affect color. Sunny days and cool nights favor anthocyanin production and bright red leaves. On cloudy days, anthocyanin isn't as chemically active and allows the orange or yellow pigments to take center stage.

Geography: Autumn leaves in Europe tend to be mostly yellow, but the US and East Asia seem to favor red leaves. Scientists from Israel and Finland recently put forth a theory about this color difference in the journal New Phytologist1. The scientists think that some 35 million years ago—amid a series of ice ages—many tree species evolved to become deciduous and produced red leaves to ward off insects. In North America and Asia, north-to-south mountain chains enabled the north and south spread of plants and animals corresponding with the advance and retreat of ice. In Europe, east-to-west mountain ranges like the Alps trapped plant and animal life. Many tree species (and the insects that depended on them) died out when the ice advanced. At the end of repeated ice ages, say the scientists, the tree species that survived didn't need red leaves to cope with the insects that were left, so they stopped producing red pigments and stuck with yellow.

The Dead Leaves and the Dirty Ground

leaves-ground

While all this color changing and autumn magic is going on, the tree is preparing to cast off its leaves. Around the same time that chlorophyll production slows down, the veins that transport nutrients and water to the leaf from the rest of the tree get closed off. A layer of cells at the base of the leaf stem, called the separation layer, swells and forms a cork-like material, gradually severing the tissue that connects the leaf to the branch. The leaf falls off and the tree seals the cut—so when the leaf is blown off or falls from its own weight, a leaf scar is left behind.

1Lev-Yadun, S and Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist Volume 183(3): 506-512. doi:10.1111/j.1469-8137.2009.02904.x
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Special thanks to Damian Dockery, who provided the foliage photos. See more of his work at flickr.com/damiand23.

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Big Questions
Why Does Santa Claus Give Coal to Bad Kids?
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The tradition of giving misbehaving children lumps of fossil fuel predates the Santa we know, and is also associated with St. Nicholas, Sinterklaas, and Italy’s La Befana. Though there doesn't seem to be one specific legend or history about any of these figures that gives a concrete reason for doling out coal specifically, the common thread between all of them seems to be convenience.

Santa and La Befana both get into people’s homes via the fireplace chimney and leave gifts in stockings hung from the mantel. Sinterklaas’s controversial assistant, Black Pete, also comes down the chimney and places gifts in shoes left out near the fireplace. St. Nick used to come in the window, and then switched to the chimney when they became common in Europe. Like Sinterklaas, his presents are traditionally slipped into shoes sitting by the fire.

So, let’s step into the speculation zone: All of these characters are tied to the fireplace. When filling the stockings or the shoes, the holiday gift givers sometimes run into a kid who doesn’t deserve a present. So to send a message and encourage better behavior next year, they leave something less desirable than the usual toys, money, or candy—and the fireplace would seem to make an easy and obvious source of non-presents. All the individual would need to do is reach down into the fireplace and grab a lump of coal. (While many people think of fireplaces burning wood logs, coal-fired ones were very common during the 19th and early 20th centuries, which is when the American Santa mythos was being established.)

That said, with the exception of Santa, none of these characters limits himself to coal when it comes to bad kids. They’ve also been said to leave bundles of twigs, bags of salt, garlic, and onions, which suggests that they’re less reluctant than Santa to haul their bad kid gifts around all night in addition to the good presents.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Who Was Heisman and Why Does He Have a Trophy?
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Brett Deering/Getty Images

On Saturday night, one of three finalists will be named this year's Heisman Trophy winner. But before anyone brings home the hardware, let’s answer a few questions about John Heisman and his famous award.

Who Exactly Was John Heisman?

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His name is mostly associated with the trophy now, but Heisman (right) was a player, coach, and hugely successful innovator in the early days of football. After playing for Brown and then Penn as a collegian from 1887 to 1891, Heisman became a coach at a series of schools that included Oberlin, Buchtel, Auburn, Clemson, Penn, Washington & Jefferson, Rice, and, most notably, Georgia Tech.

For What Football Innovations Does Heisman Get Credit?

Just some little trivial stuff like snapping the ball. Centers originally placed the ball on the ground and rolled it back to their quarterbacks, who would scoop it up and make plays. When Heisman was coaching at Buchtel (which later became the University of Akron), though, he had a 6’4” QB named Harry Clark. Clark was so tall that picking the ball up off the ground was wildly inefficient, so Heisman invented the center snap as an easy way to get the ball in Clark’s hands. Heisman also innovated the use of pulling guards for running plays and the infamous hidden-ball trick.

Any Other Shenanigans on Heisman’s Resume?

You bet. When Heisman found a way to gain an edge, he jumped on it no matter how ridiculous it seemed. When Heisman was coaching at Clemson in 1902, his team traveled to Atlanta for a game against Georgia Tech. Although Heisman was known for being a rather gruff disciplinarian, the Clemson team immediately started partying upon their arrival.

When Georgia Tech’s players and fans heard that the entire Clemson squad had spent the night before the game carousing, they prepared to coast to an easy win. When the game started, though, Clemson roared out of the gate en route to a 44-5 stomping.

How did Clemson crush Tech when by all rights they should have been ridiculously hungover? The “team” that everyone had seen partying the night before wasn’t really Heisman’s Clemson squad at all. He had sent his junior varsity players to Atlanta the night before to serve as drunken decoys, then quietly slipped his varsity team in on a morning train right before the game.

What Kind of Coach Was He?

Heisman worked as an actor in community stock theater during the summer – he consistently received rotten reviews – and allegedly spoke in a brusque, yet bizarrely ostentatious manner. Georgia Tech’s website relates a story of one of Heisman’s speeches he would break out on the first day of practice while describing a football: "What is this? It is a prolate spheroid, an elongated sphere - in which the outer leather casing is drawn tightly over a somewhat smaller rubber tubing. Better to have died as a small boy than to fumble this football."

How Did His Name Get on the Trophy?

After leaving his head-coaching job at Rice in 1927, Heisman became the athletic director at New York’s Downtown Athletic Club. In 1935 the club began awarding the Downtown Athletic Club Trophy to the nation’s top college football star. (Chicago’s Jay Berwanger won the first trophy.) Heisman died of pneumonia the following fall before the second trophy could be awarded, and the club voted to rename the prize the Heisman Memorial Trophy Award.

Did He Ever Really Throw that Iconic Stiff Arm?

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Possibly, but Heisman didn’t have the ball in his hands all that much. Even though he was a fairly small guy at just 5’8” and 158 pounds, he played as a lineman throughout his college career.

The famous “Heisman pose” is actually based on Ed Smith, a former NYU running back who modeled for the trophy’s sculptor in 1934. Interestingly, Smith went years without knowing that he’d modeled for the famous trophy. His sculptor buddy Frank Eliscu had just needed a football player to model for a project, and Smith volunteered.

Smith figured Eliscu was just doing some little personal sculpture and remained totally oblivious to his spot in football history for the next 48 years until a documentary filmmaker called Smith to interview him about the Heisman in 1982. Smith initially had no idea what the guy was talking about, but he eventually remembered his modeling days. In 1985, the Downtown Athletic Club gave Smith his own copy of the Heisman, and in 1986 he even received recognition on the televised ceremony. He looked at the four finalists – Vinny Testaverde won that year – and quipped, "Whoever wins the award, I feel sorry for you, because you're going to be looking at my ugly face for a long time." [Pictured Above: Auburn's Bo Jackson in 1985.]

What’s a Heisman Trophy Worth on the Open Market?

Quite a bit. A number of Heisman winners have eventually sold their hardware, and the trophies fetch quite a bit of loot. O.J. Simpson got $230,000 for his, and several others have gone for six-figure prices. The most expensive trophy that’s changed hands was Minnesota back Bruce Smith’s 1941 award; it fetched $395,240.

How Did Steve Spurrier Change the Process?

SEC fans are going to be floored by this one, but the Ol’ Ball Coach did something really classy when he won the Heisman in 1966. Instead of taking the trophy for himself, Spurrier gave it to the University of Florida so the school could display it and let the student body enjoy it. Florida’s student government thought Spurrier’s generosity was so classy that they paid for a replica for Spurrier so he’d get to have his own trophy, too. Since then both the school and the player have received copies of the trophy.

So Heisman Must Have Been the World’s Greatest Sportsman, Right?

Well, not really. Heisman was on the victorious side of possibly the most gratuitously run-up score in sports history. In 1916 tiny Cumberland College canceled its football program and disbanded its squad, but it had previously signed a contract to travel to Atlanta to play Heisman’s Georgia Tech team. If Cumberland didn’t show up, they had to pay Georgia Tech a $3,000 penalty, which was quite a bit of cash in 1916.

Rather than forfeiting the money, Cumberland scraped together a team of 16 scrubs and went to take their walloping from Heisman’s boys. For reasons that still aren’t totally clear – some say it was to avenge an earlier baseball loss to Cumberland, while others claim Heisman wanted to make a statement about the absurdity of the old system of using total points scored to determine the national champion – the legendary coach showed Cumberland’s ragtag band no mercy. Tech went up 63-0 in the first quarter, but Heisman kept attacking until the final score was 222-0. There are tons of hilarious stats from the game, but the funniest is Georgia Tech rushing for 1,620 yards while Cumberland only squeaked out negative-96 yards on 27 carries.

This article originally appeared in 2010.

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